Sintered ferromagnetic core having accurately adjusted dimensions

ABSTRACT

A sintered ferromagnetic coil core having accurately adjusted dimensions obtained by coating the cores in a mold with a layer of coating material consisting of a binder and a ferromagnetic powder.

United States Patent [191 Lathouwers et al.

[111 3,829,806 [451 Aug. 13, 1974 1 SINTERED FERROMAGNETIC CORE HAVING ACCURATELY ADJUSTED DIMENSIONS [75] Inventors: Franciscus Johannes Maria Lathouwers; Jacob De Groot, both of Emmasingel, Eindhoven, 1 Netherlands [73] Assignee: U.S. Philips Corporation, New

York, NY.

[22] Filed: Aug. 14, 1973 [21] Appl. No.: 388,256

[30] Foreign Application Priority Data Oct. 9, 1970 Netherlands ..70l48 l 3 Related US. Application Data [62] Division of Ser. No. 184,696, Sept. 29, 1971, Pat.

[52] US. Cl 336/83, 335/210, 336/233 [51] Int. Cl. H0lf 27/24 [58] Field of Search 336/83, 233, 212; 335/210; 29/605, 608; 117/95, 235

[56] References Cited UNITED STATES PATENTS 476,816 6/1892 Pfannkuche 336/83 683,954 10/1901 Liebreichu. 336/233 X 2,118,291 5/1938 Bollman 336/233 3,325,760 6/1967 Bernard 336/83 X FOREIGN PATENTS OR APPLICATIONS 214,780 0/1924 Great Britain 336/83 Primary ExaminerThomas J. Kozma Attorney, Agent, or Firm-Frank R. Trifari; Carl P. Steinhauser 5 7 ABSTRACT A sintered ferromagnetic coil core having accurately adjusted dimensions obtained by coating the cores in a mold with a layer of coating material consisting of a binder and a ferromagnetic powder.

4 Claims, 3 Drawing Figures PAIENTEDAUI; \3 m4 3,829.;806

SINTERED FERROMAGNETIC CORE HAVING ACCURATELY ADJUSTED DIMENSIONS This is a division of application Ser. No. 184,696, filed Sept. 29, 1971, now US. Pat. No. 3,775,841.

The invention relates to a method of accurately adf justing the dimensions of sintered ferromagnetic cores for coils or transformers.

A treatment of this kind is required if severe requirements are imposed as regards the shape and the-dimensions of the surface of the core, because the core is considerably deformed during sintering. Such severe requirements are imposed, for example, for coils whose core has to have an accurately defined air gap and for cores for deflection coil assemblies, the so-termed yoke rings, which have to fit accurately around the deflection coils.

According to a known method (see, for example, Netherlands Patent Application No. 6909344), the material of the core is locally made too thick upon sintering, after which the core is ground down to the desired dimension.

This method is comparatively expensive, particularly if the surface to be ground is not readily accessible or has a complicated shape.

The invention has for its object to provide a method by which the dimensions of cores having such a complicated surface can be controlled quickly and inexpensively. According to the invention this object is achieved in that after sintering, before the core is provided with a winding to form a coil or transformer, at least a portion of the surface of the core is coated in a mold with a layer of coating material consisting of a mixture of a binder and a ferromagnetic powder. The coating material can be pressed or injected into the mold in a known manner. The ultimate surface of the coated portion of the core is an accurate impression of the corresponding surface of the mould.

The coating material generally has a lower magnetic permeability than the sintered material of the core. This means that in coils having an air gap whose width is to be accurately defined, also the thickness and the permeability of the coating material have to be taken into account for calculating the width of the air gap. A layer of coating material having a permeability p. and a thickness d corresponds to an air gap having a width d/ p. This demonstrates that, since the coating layer is to compensate for the sintering tolerances and hence has a thickness which varies for each individual product, it is desirable for proper reproducibility that the permeability of the coating material is as large as possible. On the other hand, the quantity of binder may not be too small so as not to reduce the moldability of the coating material. A favorable compromise can be obtained by means of a coating material whose permeability has a value of approximately after hardening of the binder. Thickness variations in the coating material then correspond to width variations of the air gap which are ten times as small, which is in view of the usual tolerances of sintered cores fully acceptable for most applications.

It is to be noted that the surrounding of a coil by a moulding compound is known, for example, from Swiss Patent Specification No. 228,763 The windings of the coil, however, are then embedded in the moulding compound, after they have been provided on the core, so as to increase the inductance, thus forming an inte- FIG. 2 is a partly axial sectional view of a deflection coil assembly, the yoke ring of which has not been treated after sintering, and

FIG. 3 is a partly axial sectional view of a deflection coil assembly whose yoke ring has been given the correct dimensions after sintering by the method according to the invention.

The coil shown in FIG. 1 comprises a cylindrical winding 1 which is wound on a central cylinder 3 having connecting wires 4. This central cylinder will generally be ferromagnetic. lf the central cylinder 3 is sintered from ferromagnetic material, the dimensions of its outer surface can be controlled without much difficulty, for example, by centerless grinding. As a result, the outer diameter of the winding 1 has a small tolerance.

Provided coaxially with the central cylinder 3 is a hollow, outer cylinder 5 made of sintered ferromagnetic material, said cylinder constituting the core of the coil in conjunction with the central cylinder 3. Between the hollow outer cylinder 5 and the winding 1 is situated an air gap 6, the width tolerance of which is determined mainly by the variations possible in the inner diameter of the outer cylinder 5. In order to reduce, this tolerance, the inner side of the outer cylinder 5 is raised in a mold by providing a layer of coating material 7, consisting of a binder, for example, polythene, and a ferromagnetic powder, for example, ferrite. The layer 7 partly fills the air gap 6. Variations in the dimensions of the outer cylinder 5 merely cause thickness variations in the layer 7, without affecting the width of the air-filled portion 8 of the air gap 6. The total effective width of the air gap 6 is equal to the width of the airfilled portion 8 plus the'thickness of the layer 7, divided by the permeability of the coating material, said permeability preferably amounting to 10. This means that a variation of 0.1 mm in the thickness of layer 7, caused by the tolerance of the hollow outer cylinder 5, corresponds to a variation of only 0.01 mm in the total effective width of the air gap 6.

In order to obtain a proper magnetic connection between the central cylinder 3 and the outer cylinder 5, each of the connecting wires 4 can be secured in the (hollow) central cylinder by means of a plug 9 of the same material as the layer 7. The plug 9 can be connected to the outer cylinder 5 by means of a disc 10 which is also formed from the same material.

Providing the winding 1 on the central cylinder 3 can be facilitated by imparting a conical surface to the portion of the plug 9. extending outside the central cylinder, so that a gradual change-over from the diameter of the central cylinder to that of the connecting wire 4 is obtained.

HO. 2 shows a deflection coil assembly having a pair of saddle coils 11 for the frame deflection (the pair of line deflection coils provided within the frame coils is omitted for the sake of clarity). The coil pair 11 is surrounded by a yoke ring 13 of sintered ferrite, having a flared inner surface which has not been treated after sintering. The Figure clearly illustrates that a yoke ring 13 which has shrunk only slightly more than the ring 13, will take in a position which has shifted considerably in the axial direction.

This drawback does not occur with the yoke ring 15 shown in FIG. 3 and surrounding a similar coil assembly as in FIG. 2. The flared inner side of the yoke ring 15 is raised in a mould by coating with a layer of coating material 17 of the same composition as the layer 7 outer cylinder of sintered ferromagnetic material coaxially surrounding a cylindrical winding, said outer cylinder having a layer of coating material consisting of a ferromagnetic material and a binder on the inner surface thereof thereby at least partly filling an air gap between the core and the winding.

2. A core having accurately adjusted dimensions for an inductance coil comprising a yoke ring of sintered ferromagnetic material having a flared inner surface, and a layer of coating material consisting of a ferromagnetic material and a binder on the flared inner surface substantially masking variations in shape and dimensions of said surface 3. A core as claimed in claim 1 in which the coating material consists of a ferrite mixed with polythene.

4. A core as claimed 'in' claim 2 in which the yoke ring consists of ferrite and the coating material consists of a ferrite mixed with a binder. 

2. A core having accurately adjusted dimensions for an inductance coil comprising a yoke ring of sintered ferromagnetic material having a flared inner surface, and a layer of coating material consisting of a ferromagnetic material and a binder on the flared inner surface substantially masking variations in shape and dimensions of said surface.
 3. A core as claimed in claim 1 in which the coating material consists of a ferrite mixed with polythene.
 4. A core as claimed in claim 2 in which the yoke ring consists of ferrite and the coating material consists of a ferrite mixed with a binder. 